BOREAS Operations 1994 Final Version 2.3 (September 13, 1996) Executive Summary The design of the BOREAS-94 field experiment is described in the BOREAS Experiment plan (EXPLAN-94); preliminary results and a summary of field operations and weather conditions may be found in Sellers et al. (1995). This document describes what was actually achieved in BOREAS-94. The reports from individual investigators, minutes from evening planning meetings, aircraft logs, etc. have all been analyzed to produce a condensed history of the measurements taken in the field and the experimental conditions experienced during the field year. This document is primarily intended to serve as a desk-top guide for scientists and staff in the project which will allow quick scanning across investigations prior to delving into the detailed data sets held in BORIS. 1.0 Introduction to BOREAS The Boreal Ecosystems Atmosphere Study (BOREAS) is a large scale, international investigation focused on improving our understanding of the exchanges of radiative energy, sensible heat, water, CO2 and trace gases between the boreal forest and the lower atmosphere. A primary objective of BOREAS is to collect the data needed to improve computer simulation models of the important processes controlling these exchanges so that scientists can anticipate the effects of global change, principally altered temperature and precipitation patterns, on the biome. The scientific issues at stake are as follows: I. Sensitivity of the boreal forest biome to changes in the physical climate system. A number of simulation studies have been carried out to assess the climatic impact of increasing atmospheric CO2, see the reviews of Schlesinger and Mitchell (1987), Harrington (1987) and Houghton et al. (1990). Many of these studies indicate that the greatest warming engendered by increasing CO2 will occur at higher (45˚N-65˚N) latitudes with the most marked effects within the continental interiors; for example, the doubled-CO2 experiment of Mitchell (1983) produced differences of 3K to 10K in the mean winter surface temperature for much of the land surface area of this zone. Other studies have indicated that there may be significant warming and drying in the summer months in the same region. Studies by Davis and Botkin (1985) and Solomon and Webb (1985) suggest that this warming and drying could modify the composition and functioning of the boreal forest. 2 II. The carbon cycle and biogeochemistry in the boreal forest. The study of Tans et al. (1990) was the first to present evidence for the existence of a large terrestrial sink for fossil fuel carbon in the mid latitudes of the Northern Hemisphere. More recently, work by Denning et al. (1995) and Ciais et al. (1995), has reinforced this conclusion. The exact mechanisms involved and the spatial contributions to this sink are as yet unknown, but the implication is that carbon is being stored in either living tissue or in the soil. However, any sustained increase in surface temperature, combined with changes in soil moisture, could result in changes in the cycling of nutrients in the soils with associated releases of CO2, CH4 and other trace gases from the surface. If this occurs on a large enough spatial scale, the oxidative capacity of the lower atmosphere could be significantly altered. Additionally, changes in the temperature and moisture regime could alter the biomes' exposure and response to discontinuous disturbance, i.e. fire frequency, which could substantially affect the carbon cycle within the biome. As yet, we do not know enough about the processes which control the carbon cycle to be able to predict or even to simulate the carbon source/sink dynamics within the region. III. Biophysical feedbacks on the physical climate system. Research work has indicated (See I. above) that changes in the ecological functioning of the biome could be brought about by changes in the physical climate system. It is anticipated that these may be accompanied by alterations in the biophysical characteristics of the surface; namely albedo, surface roughness and the biophysical control of evapotranspiration (surface and internal resistance). Any changes in these may have feedback effects on the near- surface climatology (temperature, humidity, precipitation and cloudiness fields), see Sato et al. (1989) and Bonan et al. (1992, 1995). These scientific issues provided the motivation for the design and execution of a cooperative field experiment involving elements of land surface climatology, biogeochemistry and terrestrial ecology with remote sensing playing a strong integrating role. A coordinated multidisciplinary approach to the design of BOREAS was adopted from the outset to ensure the maximum benefit from each discipline's participation. 1.1 Objectives The overall goal of BOREAS is to improve our understanding of the interactions between the boreal forest biome and the atmosphere in order to clarify their roles in global change. The immediate experimental phase of BOREAS is planned to run over two to three years, 1993-1996. Obviously, this is too short a period for us to directly measure the ongoing effects of global change but it will allow us to observe important processes under a wide range of conditions so that we can develop and test key process models. The experimental strategy is specifically directed toward this: measurements will be taken throughout the annual cycle and at a variety of 'representative' sites to capture the range of significant climatic, edaphic (soil) and ecophysiological conditions to be found within the biome. Initially, these measurements will be used to improve our models and apply them over large areas to see how well we can describe the present situation. If this can be done convincingly over one or two annual cycles, then we will have more confidence in applying them as predictive tools to address the scientific issues listed above. In addition, the knowledge gained should enable us to design 3 better, more cost-effective long-term monitoring programs to track future changes in the biome. The governing objectives of BOREAS can therefore be stated as follows: (I) Improve the process models which describe the exchanges of radiative energy, water, heat, carbon and trace constituents between the boreal forest and the atmosphere. Our approach here is to measure the fluxes of energy (radiation, heat) and mass (water, CO2 and important trace gases) over a wide range of spatial scales together with observations of the ecological, biogeochemical, and atmospheric conditions controlling them. These data will be used to develop and thoroughly test process models before we apply them to the 'global change' issues described above. The initial focus will be on validation and improvement of local-scale energy balance, mass balance and biophysical process models that operate at relatively short time scales (seconds to seasons) and which are amenable to measurement within a two year field program. The results of this effort will also be useful for the study of ecosystem level dynamics and land surface/climate interactions at regional and local scales over longer time periods (years to decades). The field observations which support this model development include measurements of water, CO2 and trace gas fluxes at the plot or leaf scale (chambers, porometers), the stand scale (tower mounted devices) and the mesoscale (airborne eddy correlation). These measurements will be coordinated with a series of ecological, meteorological and edaphic observations which will link these fluxes to appropriate state variables. (II) Develop methods for applying the process models over large spatial scales using remote sensing and other integrative modeling techniques. The process studies described in (I) above have been coordinated with remote sensing investigations using satellite, airborne and surface-based instruments which focus on methods for quantifying the critical state variables. These remote sensing studies, combined with mesoscale meteorological studies, will allow us to scale-up and apply the process models at regional and ultimately global scales. Some large-scale validation techniques were incorporated in the experiment design to test our scale-integration methods directly, including airborne eddy correlation and meteorological observation and modeling. 4 1.2 Experiment Design The principal objectives of BOREAS defined in (I) and (II) above relate to two different spatial scales that must be reconciled within the experiment design. The primary focus of objective I is best addressed by local scale (a few centimeters to a few kilometers) process studies which involve detailed coordinated in-situ observations; e.g., leaf and soil plot scale, CO2 and water flux measurements and tower-mounted eddy correlation. These local-scale studies must be connected to the larger-scale measurement and analysis tools associated with objective II which is directed toward defining regional-scale (10 to 1000 kilometers) fluxes and states. In BOREAS, as in previous field experiments such as FIFE (Sellers et al.; 1992) and HAPEX-Sahel (Goutorbe et al.; 1994), the science team adopted a nested multiscale measurement strategy to integrate observations and process models over a defined range of spatial scales. At the regional scale, satellite remote sensing, meteorological observations and modeling, and airborne flux measurements provide a large-scale picture of the important processes governing the exchanges of energy, water and carbon between the atmosphere and the